Electrodeposition of chromium



atented Aug. 1, 1950 UNITED STATES ZMYAM Russell Baal), Detroit, Mich,assignor to Ductile Chrome Process 00., Detroit, Mich, a corporation ofMichigan No Drawing. Application .luly 9, 1945, Serial No. 604,093

Claims.

This invention relates to the art of chromium plating and specificallyto a new electrolytic solution from which metallic chromium can beelectaro-deposited.

It further relates to an improved electrolytic solution from which canbe electroplated a hard chromium deposit which possesses superiorqualities and better adhesion to the underlyin metal.

It is known in the art that metallic chromium deposits from conventionalelectrolytes on ferrous and nonferrous products, both for decorative aswell as wear purposes, have a disadvantage of being brittle and lackingductility. This disadvantage is shown by low resistance to impacts andstresses, because of the relatively poor bond between the chromium andthe underlying metal. For this reason, chromium deposits produced withthe conventional electrolytes have a tendency to spall or chip whensubjected to external loads or stresses.

Another disadvantage inherent in the conventional electrolytic solutionsis their inability to cover evenly, recessed areas of the part beingchromium plated. This well known lack of "throwing power has, up to thepresent time, necessitated very careful designing of anodes in order toproduce smooth and uniform deposits.

An additional inherent disadvantage of conventional electrolytes forchromium deposition is their poor current efiiciency which, undercommercial plating conditions, as stated in the literature, ranges from942% of the theoretical yield.

A further and serious disadvantage of conventional electrolytes residesin the tendency for the bath to deteriorate after a comparatively shortperiod of use and before a substantial portion of the active ingredientshave been used up. When an electrolytic bath starts to "go bad, the factis evidenced by decrease in throwing power, pimpling of the plate,non-uniform plating, discoloration of the plate and roughness. Variousmeans have been resorted to in an effort to reactivate the bath whenthis condition appears, such as addition of fresh ingredients, but thesehave been generally unsuccessful and the universal practice is to throwthe electrolyte away and make a new bath. Qbviously, this isuneconomical and Wasteful of chromium and acid.

The reason for such deterioration of the electrolyte is not fullyunderstood, but it has been observed that, in conventional baths, thepresence of trivalent chromium (CIa) is noted immediately after platingbegins in a new bath, and the percentage of Crs tends to continuallyincrease as she bath is used. When the percentage of Cm rises to about8% a noticeable decrease in plating eiiiciency and quality of the plateis evidenced and when the Cl3 content reaches about 15% in most baths,the resistance of the bath becomes so a high that plating ceasesentirely.

In my improved electrolyte, the presence of Cr: is found, but it neverexceeds 1% and tends to remain constant at less than 1% indefinitely.Plating baths made up in accordance with the 10 disclosure herein havebeen operated for months without noticeable decrease in efiiciency orthrowing power.

It is therefore the primary object of my invention to provide animproved electrolyte for chromium plating which is characterized by highthrowing power, high plating efiiciency, and inherent stability whencompared to conventional electrolytes.

Another object is to provide an improved process of chromium plating bymeans of which a plate of greater ductility and superior resistance toshock, spelling and wear is produced.

A still further object is to provide an improved process forelectrodepositing chromium whereby a markedly stronger bond between thechromium layer and the underlying metal is achieved.

Other objects and advantages will be apparent from the followingdisclosure.

My new electrolyte is composed of an aqueous solution of chromiumtrioxide (CrOs) to which is added an efitective amount of complexcompounds of chromium and organic acids containing hydroxy groups suchas: tartaric and/or malic together with hydrofluoric acid. This com.-

plex is the active agent which permits the better deposition of chromiumwhen electric current is passed through the electrolyte in accordancewith my invention.

For the purpose of this invention it is not neen essary to add thecomplex to the aqueous solution of chromium trioxide. Equally goodresults can be produced and maintained by adding the chemical reagentsinvolved directly to an aqueous solution of chromium trioxide.

5 The exact structure of the chromium complex is assumed to comprise acentral chromium atom with which are coordinated fluorine radicals aswell as radicals derived from the tartaric and/or malic acid, possiblyby oxidation, said complex possessing the ability to carry electricalcharges during the process of electrodeposition, and being able at thecathode to maintain a condition necessary to deposit chromium in thisimproved manner.

In the formation of the complex, the chromium.

hydrofluoric acid and the tartaric and/r malic acid are believed tocombine in one or more of the following ratios:

lto itol 1to3to1 lto2to2 ltoltoll 1to5to It is further believed that areversible reaction is established by which, while the complex is brokendown at the cathode, more of it is formed in other parts of the cell,and thus the quantities of the active agent involved in the process arerelatively small.

For commercial use, I have found that to an aqueous solution of chromiumtrioxide (CIOs) containing from 870 oz. per gallon of solution, must beadded from .001-1 oz. per gallon of tartaric or malic acid, and from .1to 3 oz. per gal lon of hydrofluoric acid.

I have also found that the current densities that can be used may varyfrom 50 to 4,000 amps. per sq. ft. of plated area, and that thetemperature may vary between 60 F. and 170 F.

For the best results I have found that the electrolytc should be made upto contain the following quantities: (CrOa) chromium trioxide, 36-60 oz.per gal.; tartaric or malic acid .00l-.25 oz. per gal.; hydrofluoricacid .5-2.0 oz. per gallon.

The best operating conditions will be temperature ll0-140 F., withcurrent densities from 200-1000 amps. per sq. ft. of plated area.

A bath made up and used as described above will operate indefinitely.The trivalent chromium content remains constant at a value of less than1 and the current and plating eihcienc remains uniformly high.Deterioration of the bath may be quickly detected by slight treeing ofthe plate and when this occurs, the addition of a small amount of thecomplex will instantly restore the bath. In most instances, hydrofluoricacid alone is added, the tartaric or malic acid apparently remaining inthe bath as an active ingredient without deterioration.

By the use of this electrolyte, I have found that the throwing power isgreatly increased, and uneven surfaces can be plated with simple anodesand with greater uniformity in the thickness of the deposit, andprimarily smoother coating in the places of high current densities, suchas: corners or protruding areas.

I have also found that, when above optimum conditions are maintained,the rate at which chromium can be deposited is considerably greater thanthat which has been reported for the conventional electrolytes.

The great advantage possessed by chromium deposits from the newelectrolyte is the unusually strong bond existing between the chromiumlayer and the underlying metal as is shown in the conventional bendtest. This test reveals that, in conventional chromium plated depositsthe cracks produced by the bending stresses are very numerous anddistributed at random indicating a brittle deposit, while the depositsfrom my new electroiyte, the cracks are considerably fewer anddistributed in the direction of the maximum stresses.

It is for this reason that chromium plating deposits from the newelectrolyte are considerably more resistant to shocks and wear, and donot chip under load.

The new electrolyte has been successfully used in plating of dies,reamers, drills, gages, and a great variety of tools in which uniformthickness of plating as well as evenly covered recessed areas are ofprimary importance.

For the purpose of this patent, I do not wish to limit the scope of myinvention to the above named objects, because the electrolyte can beused to plate any form or type of tools and any other object which mayrequire chromium plating for wear, abrasion, and corrosion resistance.

I claim:

1. A plating bath for electrolytically depositing chromium, beingcharacterized by superior throwing power, high current efiiciency andsubstantially constant low concentration of trivalent chromium,consisting essentially of an aqueous solution containing per gallon ofsolution from about 8 to oz. chromic oxide, from about 0.1 to 3 oz.hydrofluoric acid and from about 0.001 to 1 oz. of an organic acidselected from a class consisting of malic and tartaric acids.

2. A plating bath for electrolytically depositing chromium, beingcharacterized by superior throwing power, high current eihciency andsubstantially constant low concentration of trivalent chromium,consisting essentially of an aqueous solution containing per gallon ofsolution from about 36 to 60 oz. chromic oxide, from about 0.5 to 2 oz.hydrofluoric acid and from about 0.001 to 0.25 oz. 01' an organic acidselected from a class consisting of malic and tartaric acid.

3. An electroplating process which comprises electrodepositing chromiumfrom a plating bath consisting essentially of an aqueous solutioncontaining per gallon of solution from about 8 to 70 oz. chromic oxide,from about 0.1 to 3 oz. hydrofluoric acid and from about 0.001 to 1 oz.of an organic acid selected from a class consisting of malic andtartaric acids.

4. An electroplating process which comprises electrodepositing chromiumfrom a plating bath consisting essentially of an aqueous solutioncontaining per gallon of solution from about 36 to 60 oz. chromic oxide,from about 0.5 to 2 oz. hydrofluoric acid and from about 0.001 to 0.25oz. of an organic acid selected from a class consisting of malic andtartaric acids.

5. In the electrolytic plating of chromium with a plating bathconsisting essentially of an aqueous solution containing from about 8 to'70 oz. chromic oxide per gallon, hydrofluoric acid and an organic acidselected from a class consisting of malic and tartaric acids, theprocess of preventing the accumulation of trivalent chromium in the bathwhich consists in maintaining a sufficient quantity of hydrofluoric acidand organic acid in the bath to prevent the concentration of trivalentchromium in the bath from rising substantially above 1 per cent byweight, whereby the life of the plating bath can be extendedsubstantially indefinitely.

RUSSELL RAAB.

CES CETED The following references are of record in the file of thispatent:

UNITED STATES PATENTS Number Name Date Re. 16,598 Hambuechen Apr. 19,1927 526,114 Placet et al Sept. 18, 1894 i,5i2,5 l9 Grah June 16,19251,923,284 Fink Sept. 26,1933 2,114,002 Kramer Apr. 12,1938 2,307,551'Iriska Jan. 5, 1943 Certificate of Correction Patent No. 2,517,441August 1, 1950 RUSSELL RAAB It is hereby certified that error appears inthe above numbered patent requiring correction as follows:

In the grant, lines 2 and 3, and in the heading to the printedspecification, lines 4 and 5, for a corporation of Michigan read acompany of Michigan;

and that the said Letters Patent should be read as corrected above, sothat the same may conform to the record of the case in the PatentOflice.

Signed and sealed this 7th day of November, A. D. 1950.

[men] 'IHQMAS F. MURPHY,

Assistant Uommissz'oner of Patents.

1. A PLATING BATH FOR ELECTROLYTICALLY DEPOSITING CHROMIUM, BEINGCHARACTERIZED BY SUPERIOR THROWING POWER, HIGH CURRENT EFFICIENCY ANDSUBSTANTIALLY CONSTANT LOW CONCENTRATION OF TRIVALENT CHROMIUM,CONSISTING ESSENTIALLY OF AN AQUEOUS SOLUTION CONTAINING PER GALLON OFSOLUTION FROM ABOUT 8 TO 70 OZ. CHROMIC OXIDE, FROM ABOUT 0.1 TO 3 OZ.HYDROFLUORIC ACID AND FROM ABOUT 0.001 TO 1 OZ. OF AN ORGANIC ACIDSELECTED FROM A CLASS CONSISTING OF MALIC AND TARTARIC ACIDS.